Metal halide lamps are known in the art and are described, for example, in EP0215524 and WO2006/046175. Such lamps operate under high pressure and comprise ionizable gas fillings of, for example, NaI (sodium iodide), TlI (thallium iodide), CaI2 (calcium iodide), and/or REIn. REIn refers to rare earth iodides. Characteristic rare earth iodides for metal halide lamps are CeI3, PrI3, NdI3, DyI3, and LuI3.
An important class of metal halide lamps are ceramic discharge metal halide lamps (CDM-lamps), which are described in the above-mentioned documents. The ionizable fillings (comprising rare earth salts) in the discharge vessels of such lamps are added in amounts that lead to a saturated vapor when the discharge lamp is operated, thereby leaving part of the filling in a condensed phase. A reason for adding the filling in an amount that will lead to a saturated vapor during use of the lamp may be the fact that salts may react with the discharge vessel wall and/or other elements within the discharge vessel during use, which leads to a reduction in the amount of filling. Hence, when aiming at a discharge lamp with a constant output, providing a saturated gas filling seems a prerequisite.
The lamps described in e.g. EP0215524 are said to provide a high luminous efficacy and a satisfactory color rendering. It is described that the discharge vessel comprises at least one halide of at least one of the elements Sc, La and the lanthanides; the elements Dy, Tm, Ho, Er and La being preferred. The examples describe discharge vessels with about 18.2-21.8 mg/cm3 mercury and about 11.2-14.7 mg/cm3 NaI, TlI and DyI3. The halides are in excess; i.e. unevaporated sodium halide is still present during operation of the lamp. The coldest spot temperature is, for example, about 900° C. (1173 K).